Due to the harmful effects of lead on the human body, the use of lead-containing solders is now regulated. For this reason, Sn-based, lead-free solders have become widely used. At present, Sn-3Ag-0.5Cu solder is much used as a lead-free solder in so-called household electronic equipment such as televisions, video cameras and decks, mobile telephones, and personal computers. This lead-free solder is somewhat inferior to conventional Pb—Sn solders with respect to solderability. However, as a result of improvements in flux and soldering apparatuses, it can now be used without problems, and it does not experience peeling or similar problems in ordinary use over the lifespan of household electronic equipment.
A heat cycle test is used to test the durability of soldered joints in household electronic equipment. In a heat cycle test which is widely used for household electronic equipment, chip resistor components measuring 3.2×1.6×0.6 mm are soldered to a printed circuit board, and a heating and cooling cycle in which soldered joints are held at a high temperature of +85° C. and then a low temperature of −40° C. for 30 minutes at each temperature is repeated for 500 cycles. Subsequently, the state of electrical conduction of the soldered joints is measured. The result is acceptable if conduction takes place.
Electronic circuits having electronic parts soldered to, i.e., mounted on a printed circuit board are also mounted on automobiles (such circuits are referred to below as vehicle-mounted electronic circuits). Heat cycle tests are also carried out on vehicle-mounted electronic circuits. A heat cycle test used for vehicle-mounted electronic circuits is carried out under extremely severe test conditions which, as described later, are far more severe than those of the heat cycle test for household electronic equipment.
There have been many proposals in the past of lead-free solders having excellent resistance to heat cycles. See Patent Documents 1-3.
However, none of them exhibited sufficient resistance to heat cycles when tested by a heat cycle test of the type which is currently required for soldered joints of vehicle-mounted electronic circuits.
Patent Document 1: JP 05-228685 A1
Patent Document 2: JP 09-326554 A1
Patent Document 3: JP 2000-349433 A1